This invention relates to single crystal alloys for high stress, high temperature application under conditions of long and continuous operation.
Alloys of this type are particularly essential in turbine blade technology in which resistance to oxidation and high temperature corrosion combined with high thermal fatigue and high creep strengths are essential. These alloys are commonly referred to as "superalloys".
The concept of alloys for casting single crystal parts was developed several years ago and its development can in part be followed in U.S. Pat. Nos. 3,494,709, 4,116,723 and 4,209,348. In addition to the alloys, process technology has been developed for the casting of single crystal alloy parts which prevent the growth of all grain structures except one. The techniques involved in such technology are not part of this invention, rather the invention being the development of an alloy which can be used with such techniques.
In the making of single crystal alloy parts, the part is first cast from an alloy suitable for single crystal technology. Process methods are employed during casting solidification to ensure only one grain forms in the mold cavity, which in fact, forms the part being cast such as a turbine blade. The resulting single crystal casting will, however, not be homogeneous because of substantial "coring" [dendritic segregation] of some of its elements. To eliminate this condition, the casting is subsequently heat treated. In so doing the casting must be heated to a sufficiently high temperature where all of the primary gamma prime strengthening precipitate and some of the eutectic goes into solution. This temperature, however, must be below that at which the casting starts to melt, i.e. the incipient melting point. The temperature range at which this can be accomplished may be called the "heat treatment window". The wider the "window", the greater the possibility that the heat treatment will be successful in solutioning all the primary gamma prime and some of the eutectic without incipient melting occurring. To the extent that there is a failure to solution all of the primary gamma prime in the final casting, there will be a reduction in high temperature strength. If the incipient melting point is reached, there will be a loss of ductility and likelihood of premature creep or fatigue failure due to void formation.
The single crystal alloys heretofore developed, have all had undesirable characteristics. If they had the desired resistance to high temperature fatigue and creep coupled with good oxidation and corrosion resistance, they had such a narrow heat treatment "window" that they were virtually impractical to heat treat on a production basis and lacked dependable repeatability. When it was attempted to widen the heat treatment "window", one of the alloys developed high temperature phase instability which resulted in the formation of plate-like/acicular phase of alphatungsten, alphachromium and mu phase. This has been found to be a progressive condition resulting in deterioration of long time creep and fatigue properties, once the formation of these conditions has been initiated.